1. Field of the Invention
The present invention relates to a high variable power zoom lens suitable for a single-lens reflex camera, and more particularly, it relates to a compact and lightweight, high variable power zoom lens that is capable of having a sufficient back focal length, suitable for a single-lens reflex camera, and advantageous in incorporating optical stabilizer or anti-vibration mechanism, and that is 75 degrees or even wider in field angle at the wide-angle end, approximately 3 to 4 in F-number at the wide-angle end and approximately 6 to 7 at the telephoto end, and about 15× in variable power ratio.
2. Description of Related Art Including Information Disclosed Under 37 CFR §§1.97 and 1.98
The recent drastic advancement in technologies of optical designs and manufacturing optics has enabled zoom lenses to be downsized more and enhanced in variable power. For instance, one type of the zoom lenses in the state of the arts have a variable power enhanced design and enable to incorporate an optical stabilizer or anti-vibration feature, which typically comprise four groups of lens pieces, namely, the 1st lens group of positive refractivity, the 2nd lens group of negative refractivity, the 3rd lens group of positive refractivity, and the 4th lens group of positive refractivity arranged in this order on the closer to objects first basis where the power is varied from the wide-angle end to the telephoto end as a result of the 1st and 2nd lens groups being separated more, the 2nd and 3rd lens groups coming closer to each other, and the 3rd and 4th lens groups being varied in a distance therebetween. The 3rd lens group has a leading set of the lens pieces closer to the object side having a positive refractivity as a whole and a trailing set of the lens pieces having a negative refractivity as a whole, and the trailing set alone is moved in position approximately orthogonal to the optical axis so as to compensate for defocus in the imaging plane caused by a tremor of the user's hand(s), which can be corrected by the anti-vibration feature of which requirements are to satisfy the following formula:3.5<f1/fw<8.0where fw is a focal length of the zoom lens at the wide-angle end, and f1 is the focal length of the 1st lens group (see Patent Document 1 listed below).
Another type of the high variable power zoom lenses are compatible with an APS-C dimensioned image sensor and are dedicated to digital cameras, which also have four groups of lens pieces, namely, the 1st lens group of positive refractivity, the 2nd lens group of negative refractivity, the 3rd lens group of positive refractivity, and the 4th lens group of positive refractivity arranged in this order on the closer to objects first basis where the power is varied from the wide-angle end to the telephoto end as a result of the 1st and 2nd lens groups being separated to elongate an aerial distance therebetween, and the 2nd and 3rd lens groups coming closer to each other while simultaneously the 1st, 3rd, and 4th lens groups are moved all together closer to objects. The 2nd lens group are displaced for the focusing of which requirements are to satisfy the following formulae:0.40<fW/fbW<0.55  (1)0.43<β34W/β34T<0.47  (2)40<r5/d5W<100  (3)1.1<f3/f4<2.6  (4)where fW is a focal length of the zoom lens at the wide-angle end, fbW is a back focal length at the wide-angle end, β34W is a composite power of the 3rd and 4th lens groups at the wide-angle end, β34T is the composite power of the 3rd and 4th lens groups at the telephoto end, r5 is a radius of curvature of the surface designated by r5, d5W is a distance between the 1st and 2nd lens groups at the wide-angle end, f3 is the focal length of the 3rd lens group, and f4 is the focal length of the 4th lens group. This type of the zoom lenses are, regardless of their original design concept especially suitable for the APS-C size, able to ensure the same back focusing as those for image dimensions for 3.5 mm film (see Patent Document 2).